Wireless communications includes a large number of applications that service a wide variety of communication needs. Different communication markets are characterized by different transmission protocols and frequency bands. These markets are encumbered by technology fragmentation resulting from competitors who have a vested interest in promoting their own proprietary transmission protocols and signal-processing technologies. This fragmentation impedes compatibility between different applications and systems, reduces bandwidth efficiency, increases interference, and limits the usefulness of wireless communications. Thus, there is an overwhelming need to unify these technologies.
Throughout history, the quest to understand the universe has focused on discovering the elementary components of the universe. Knowledge of the properties of fundamental elements can provide an understanding of the properties of complex combinations of those elements. From an engineering perspective, the properties of a complex combination of elements are determined by properties of the elements and the manner in which the elements are combined.
Many aspects of conventional Quantum theory, as well as more recent discoveries in high-energy physics, indicate that a wave-based phenomena is the fundamental basis of all matter and energy. Quantum theory also describes a complex state as a superposition between component waveforms, the superposition resulting from constructive and/or destructive interference between the waveforms.
The idea of using multiple low-rate communication channels to transmit a large amount of data is well known. U.S. Pat. No. 5,960,032 describes dividing a high-rate data stream into a plurality of parallel low-rate bit streams that are each modulated with a direct-sequence spreading code. Other methods of multicarrier processing are described in U.S. Pat. No. 6,061,405 and U.S. Pat. No. 5,729,570. Although several prior-art methods involve redundantly modulating multiple component waveforms, none of these methods achieve the benefits of the present invention that are enabled by interferometrically combining the waveforms. For example, U.S. Pat. Nos. 5,519,692 and 5,563,906 describe geometric harmonic modulation (GHM) in which preamble and traffic waveforms are created from multiple carrier frequencies (tones). GHM waveforms comprise tones incorporating a binary phase code where signal phases are 0 or −π/2. The binary phase offsets, which are applied to the tones, provide the spreading codes. Orthogonality of GHM signals is realized upon correlation with a reference signal at a receiver. A preamble carrier waveform is constructed by summing the tones. Therefore, the preamble signals are similar to Multicarrier CDMA (MC-CDMA) signals.
Each receiver monitors the preamble signals for its own phase code and then despreads and decodes the appended traffic waveforms. The traffic waveforms are products of the tones. The receiver generates a reference waveform from a product of tones having phase offsets that correspond to the receiver's phase code. The reference waveform is correlated with the received signals to produce a correlation result that is integrated over the data-bit duration and over all tones.
GHM uses binary phase offsets instead of incremental poly-phase offsets. Thus, GHM does not provide carriers with phase relationships that enable the superposition of the carriers to have narrow time-domain signatures. Consequently, received GHM signals require processing by a correlator, whereas signals that are orthogonal in time can be processed using simpler signal-processing techniques, such as time sampling and weight-and-sum. Furthermore, GHM does not achieve the capacity and signal-quality benefits enabled by time-orthogonal signals.
U.S. Pat. No. 4,628,517 shows a radio system that modulates an information signal onto multiple carrier frequencies. Received carriers are each converted to the same intermediate-frequency (IF) signal using a bank of conversion oscillators. The received signals are then summed to achieve the benefits of frequency diversity. In this case, frequency diversity is achieved at the expense of reduced bandwidth efficiency. The process of converting the received signals to the same frequency does not allow orthogonality between multiple information signals modulated on the same carriers.
In order to accommodate the processing speeds of conventional signal-processing techniques, high-frequency carrier signals are typically down converted to an IF before demodulation. In conventional receivers, components in the IF sections comprise the majority of components of the receiver.
Conventional down converters include electrical components whose properties are frequency dependent. Consequently, conventional down converters are designed to operate at specific frequencies or frequency bands and do not have flexibility to adapt to different frequencies.
Conventional down converters employ mixers, which generate undesired intermodulation and harmonic products. Filters are needed to remove the undesired signals. Such filters reduce the power level of the modulated carrier signals and, thus, require amplifiers and additional power sources for the amplifiers.
It is preferable to reduce the number of filters and mixers in a wireless system because these components attenuate desired signals and require additional low-noise amplifiers to compensate for the reduced signal strength. Low-noise amplifiers require substantial power to operate. High-frequency amplifiers typically require more power than low-frequency amplifiers. In a portable system, such as a cellular telephone, low-noise amplifiers use a substantial portion of the system's power.
Since many radio-frequency (RF) components, such as amplifiers, filters, and impedance-matching circuits are highly frequency dependent, receivers that are designed for one frequency band are usually not suitable for applications that make use of other frequency bands. Similarly, receivers designed for a particular transmission protocol are typically not adaptable to other protocols. Furthermore, receivers are typically not adaptable to variations of the protocol for which they are designed.
Conventional receiver components are typically positioned over multiple integrated-circuit (IC) substrates to accommodate processing in RF, IF, and baseband frequencies. Receivers that use multi-mode processors (i.e., processors having separate systems designed to process different transmission protocols) use multiple ICs. Additional signal amplification is often required when bridging multiple chips. Thus, the use of multiple substrates introduces additional costs beyond the costs associated with producing the ICs.
What is needed is an underlying signal architecture and signal-processing method that not only enhances signal quality and system capacity, but also simplifies transmission and reception of communication signals. Accordingly, it is desirable that a proposed signal-processing method eliminate the need for IF processing and, thus, substantially reduce the number of components in a receiver. It is preferable that a proposed signal-processing technique enable parallel processing, adaptability to different frequency ranges, compatibility with different transmission protocols, interference mitigation, and reduced distortion.
In commercial telecommunication systems, it is well known that technology complexity leads to higher manufacturing costs, reduced reliability, and longer development cycles. For example, while IS-95 provided the highest spectrum efficiency of second-generation mobile systems, it also incurred higher costs and a longer development time to provide forward error correction, Rake receivers, power control, and soft handoff. Accordingly, it is preferable that a proposed communication system enable simple signal-processing methods and systems for transmission and reception. It is only through a simple, yet elegant signal processing technique that all of the needs discussed herein can be addressed without compromise.